Assessment Comments

Assessment is based on: (1) the results of IDNR/UHL ambient monthly monitoring during the 2004-2006 assessment period at STORET station 10910001 (formerly station 200551) at the USGS gauging station approximately 1.5 miles west of Highway 65-69 near Indianola and (2) results of IDNR/UHL biological monitoring conducted in 2002 as part of the REMAP project.

Basis for Assessment

[Note:  Prior to the current (2008) Section 305(b) cycle, this river segment was designated only for Class B(WW) aquatic life uses, including fish consumption uses.    Due to changes in Iowa’s surface water classification that were approved by U.S.  EPA in February 2008 (see http://www.iowadnr.com/water/standards/files/06mar_swc.pdf) and due to the completion of a Use Attainability Analysis in 2007, this segment is also now designated for Class A1 (primary contact recreation) uses.   This segment remains designated for warmwater aquatic life use (now termed Class B(WW1) uses), and for fish consumption uses (now termed Class HH (human health/fish consumption uses).]

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" due to levels of indicator bacteria that violate state water quality criteria.   The Class B(WW1) aquatic life are assessed (monitored) as "partially supported" based on results of IDNR/UHL biological monitoring in 2002.   Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this stream segment.   The sources of data for this assessment include (1) the results of IDNR/UHL ambient monthly monitoring during the 2004-2006 assessment period at STORET station 10910001 (formerly station 200551) at the USGS gauging station approximately 1.5 miles west of Highway 65-69 near Indianola and (2) results of IDNR/UHL biological monitoring conducted in 2002 as part of the REMAP project.  

EXPLANATION:  The Class A1 uses are assessed (monitored) as "not supported" based on results of ambient monitoring for indicator bacteria (E.  coli).   Due to recent changes in Iowa’s Water Quality Standards, Iowa’s assessment methodology for indicator bacteria has changed.   Prior to 2003, the Iowa WQ Standards contained a high-flow exemption for the Class A criterion for indicator bacteria (fecal coliforms) designed to protect primary contact recreation uses:  the water quality criterion for fecal coliform bacteria (200 orgs/100 ml) did not apply "when the waters [were] materially affected by surface runoff."  Due to a change in the Standards in July 2003, E.  coli is now the indicator bacterium, and the high flow exemption was eliminated and replaced with language stating that the Class A criteria for E.  coli apply when Class A1, A2, or A3 uses “can reasonably be expected to occur.”  Because the IDNR Technical Advisory Committee on WQ Standards could not agree on what flow conditions would define periods when uses would not be reasonably expected to occur, all monitoring data generated for E.  coli during the assessment period, regardless of flow conditions during sample collection, will be considered for determining support of Class A uses for purposes of Section 305(b) assessments and Section 303(d) listings.  

The geometric mean level of indicator bacteria (E.  coli) in the 24 samples collected (268 orgs/100ml) exceeds the Iowa Class A1 water quality criterion of 126 orgs/100ml.   Ten of the 24 samples (42%) exceed Iowa’s single-sample maximum criterion of 235 orgs/100 ml.   According to U.S.  EPA guidelines for Section 305(b) reporting and according to IDNR’s assessment/listing methodology, if the geometric mean level of E.  coli is greater than the state criterion of 126 orgs/100 ml., the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35of U.S.  EPA 1997b).  

The Class B(WW1) aquatic life uses remain assessed (monitored) as “partially supported” based on results of IDNR/UHL biological monitoring in 2002 conducted as part of the REMAP project.   A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biological sampling data.   The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa and fish species that were collected in the stream sampling reach.   The biological metrics were combined to make a fish community index of biotic integrity (FIBI) and a benthic macroinvertebrate index (BMIBI).   The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).   The 2002 FIBI scores were 21 (poor), 38 (fair) and the FIBI average was 29.5.   The 2002 BMIBI scores were 40 (fair), 24 (poor) and the BMIBI average was 32.   The aquatic life use support was assessed as Partially Supporting (=PS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established for previous Section 305(b) reports.  The biological impairment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2004.   The FIBI BIC for this ecoregion is 36 and the BMIBI BIC for this ecoregion is 51.

Previous assessments of this stream segment have indicated potential impacts due to alteration of aquatic habitats.   Thus, problems with habitat quality may explain the relatively poor IBI values for fish and aquatic macroinvertebrates.

As part of the REMAP project, diurnal dissolved oxygen/temperature monitoring conducted during July, September, and October 2002 found substantial fluctuation of dissolved oxygen levels and temperature, including nighttime sags in dissolved oxygen (<5 mg/L) and daytime temperatures exceeding 35 C (95 F).   These conditions were associated with very high estimated levels of community respiration and primary production.   Levels of BMIBI metrics that are sensitive to organic enrichment suggest a nutrient enrichment/algal growth-related water quality problem.   Habitat modification has resulted in a wide and shallow stream channel subjected to high solar input.   These physical factors probably contribute to excessive levels of algal growth, and extreme dissolved oxygen and temperature fluctuations.   Additional sampling data are needed to evaluate the extent, causes and sources of this water quality concern.

Despite results of biological monitoring which suggest impairment, results of IDNR/UHL ambient chemical/physical monitoring do not suggest an impairment.   None of the 36 samples collected during the 2004-2006 assessment period at the IDNR monthly station violated Class B(WW1) water quality criteria for dissolved oxygen, pH, or ammonia-nitrogen; no violations occurred in the seven samples analyzed for pesticides.   A single violation of the Class B(WW1) chronic criterion for lead occurred in the sample collected on July 6, 2005.   This sample contained 70 ug/l of total lead, thus exceeding the Class B(WW1) chronic criterion of 30 ug/l.   This single violation of a water quality criterion for a toxic parameter, however does not indicate impairment of the aquatic life uses:  according to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b, page 3-18), one violation of a water quality criterion for a toxic pollutant in an abundant data set (at least 10 samples over at three-year period) does not indicate an impairment of aquatic life uses.  

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this stream segment.